A Palette of Breeding Systems
breeding, by itself, does not have to narrow the diversity of the
gene pool across the full spectrum of the breed. In an extremely
simplified picture (do not take literally), for purposes of illustration
only, imagine 50 different breeders each line breeding within a different
group of loosely related dogs. These 50 breeders, for purposes of this
illustration, are not breeding repeatedly to the same stud dog within
the loosely related group but are fully utilizing all relatives that
meet health and temperament requirements. Each of these 50 breeders are
breeding within groups of stock that are not closely related to any of
the other 49 breeders' groups. This would allow each breeder to "go
out" on occasion to a dog from one of the other 49 groups of
linebred animals without loosing the benefits from line breeding.
If it helps to imagine each breeders "group" as a slightly different hue from every other breeder's group, then you can see in your mind's eye a full palette (i.e. diversity breed wide) of many colors. An occasional mixing (outcross) of a small percentage of another color on the palette will not be enough to dilute a given hue to the point where it will be transformed into one of the other distinct colors. Add to this the idea that the artist wants to keep each color balanced to remain distinct; and thus if a light color is added at one point, then the next time a dark color might be added so as to not change the hue dramatically over time. You can see that the palette does not lose colors (diversity) under this system.
Conversely, imagine what happens if ALL 50 breeders are ONLY out crossing, and especially if ALL choose to breed to one of the three or four top-winning and frequently used stud dogs. Let's consider the extreme case that each of the 50 breeders has two bitches, each bred to various of the four stud dogs. With an average litter size of five, the 50 breeders will have produced 500 puppies. But the effective gene pool population of this out crossing is not 500. Following the genetic convention that the effective population of a group of dogs cannot be more than 4 times the number of stud dogs used, the 500 puppies in this extreme example of out crossing would represent an effective population of only 16, or 4 times the 4 stud dogs used. [This is a far cry from an effective population of 200 if each of the 50 breeders line-bred to their own stud dog (4 times the 50 stud dogs used)].
If this pattern of out crossing to the same few stud dogs prevails, in a few short generations the ENTIRE (emphasizing for purposes of illustration, remember, do not take literally) gene pool has now been narrowed tremendously (i.e. all the hues are mixed together and thus one would eventually end up with all black or dark colors). If a serious genetic health threat occurs, where do the 50 breeders now go to find unrelated animals to breed to? (or, to extend the metaphor, how can they lighten the hues if all available paint colors are dark?)
I recommend the book "Genetics of the Dog" by Malcolm B. Willis. One quote that might be appropriate to this discussion:
"Lush (1945), one of the world's leading population geneticists, held that 'more opportunities for breed progress are lost by not inbreeding when inbreeding would be advisable than are lost by too much inbreeding'. This is not a carte blanche invitation to inbreed at each and every opportunity but emphasizes that inbreeding dogs does have virtues that can be exploited on occasion. It must not be forgotten that anyone getting into trouble in an inbreeding programme can, with one outcross bring inbreeding to zero whereas a breeder with an outbred programme who runs into trouble has no logical direction in which to turn for help."
While it is true that one linebreeds to "set type" (visualized as a particular and unique hue), this does not have to be translated into the homogeneity of recessives but rather an overall homogeneity of genes producing desired attributes regardless of whether or not they are recessive. This increases predictability. When one speaks of increasing homogeneity, this should not be envisioned as a single gene to single gene match but rather as an overall homogeneity. After all, many attributes involve multiple sites and require coordinated combinations among sites to produce the desired results.
Line breeding, by itself, does not have to narrow the diversity of the gene pool across the full spectrum of the breed and, used properly, is an excellent tool to set breed type.
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